An asteroid over a kilometer across has just become the second Earth Trojan asteroid, an asteroid that shares its path with Earth.

Kinda. I will explain if you bear with me for a second.

The Pan-STARRS survey found the asteroid 2020 XL5 in 2020 in images taken of the sky.

2020 XL5 has a slightly tilted elliptical path that is about 14 degrees away from Earth. Although it gets as close to the Sun as 90 million km, its semi-major axis is the same as Earth. Its year is almost the same as Earth's. It goes around the Sun in the same amount of time as the Earth.

That is important! There are several stable points along the path of a star and a planet, which were discovered by two mathematicians in the 18th century. If you place an object there, it will tend to stay there.

One in between the planet and the star, one on the opposite side of the planet, and two more behind the planet in its path. If you poke an object hard enough, it will move away, but L4 and L5 are more stable. If you put something in those spots, it will stay at or near that point.

Jupiter has the most mass of any planet, it has stable L4 and L5 points, and a whole fleet of asteroids shares the same path. The first few of these were named after figures in the war, so we call them L4 and L5 the Trojan Points, with L4 called the leading point and L5 the trailing point.

Only one EarthTrojan asteroid has been found, which is 400 meters across. It was found by mistake. These asteroids are difficult to find because they are so low in the sky that they can only be seen after sunset or before sunrise. The OSIRIS-REx mission looked for L4 Earth Trojans in space, but didn't find any.

2020 XL5 is about 1.2 kilometers wide. The size of a small asteroid is hard to get because they appear as dots in our telescopes. A small shiny asteroid is just as bright as a bigger dark one, so we have to judge their size based on how bright they appear, how far away they are, and how reflective they are.

Astronomers were able to calculate its position over time by looking for older observations taken of it before it was discovered, called precovery images. The longer the baseline of observation times, the better.

There is a place where things get fun, and it is here that things get fun. Both the Sun and Earth have periods of one year because they both have semi-major axes. Both are always ahead of Earth in their elliptical paths around the Sun, and sometimes get a bit closer and a bit farther from Earth. Both travel around Earth's L4 point. If you were above the solar system looking down on Earth and stayed directly above Earth, you would see both asteroids make a pattern over the course of a year.

Orbital mechanics is a bit weird, but it is. They never get very close to Earth, though their distances do vary.

These kinds of circles are not always stable. The asteroids can be sent off on a different trajectory if the gravity of Venus and other planets tugs on them. The simulations show that 2010 is unstable over a period of around 15,000 years, so it must have moved into this current path less than 2,000 years ago.

2020 XL5 probably became an EarthTrojan about 600 years ago, and should remain there for another 5,000 years before being kicked into a different path.

Earth Trojans are desirable space probe targets because of their low speed. If you get a probe away from Earth, you don't need much fuel to get there, and it would be easier to scoop up samples and return them to Earth.

It is much harder to get to them because of the tilted orbits. The discovery of XL5 is intriguing. It suggests that there are still largish rocks out there waiting to be discovered. The door is open for future missions if we visit a few asteroids up close.

Given the size of these rocks, it's good that they aren't close enough to be a threat.